1. Field of the Invention
The present invention relates to a thin film transistor and a method of producing the same.
2. Description of the Related Art
In recent years, thin film transistors (TFTs) using a polycrystalline silicon (hereinafter referred to as "poly-Si") film, i.e., poly-Si TFTs, have been employed as active circuit elements in LCDs, SRAMs, etc. There have been various reports on methods of forming a TFT using a poly-Si film subjected to laser crystallizing treatment as an active layer.
The method that employs laser crystallizing treatment enables a TFT to be formed on a low-cost glass substrate by a low-temperature process. With a method wherein an amorphous silicon layer is crystallized by using excimer laser light, crystal grains cannot be grown larger than about 10 nm, and hence the number of grain boundaries is undesirably large. Therefore, the crystallized silicon layer is subjected to hydrogenating treatment so that dangling bonds, which are principal defects present at the grain boundaries, are terminated by hydrogen, thereby removing the dangling bonds.
The conventional method will be explained below with reference to FIG. 6, which shows the process sequence for producing a TFT.
As shown in FIG. 6A, a glass substrate 111 has a gate 112 provided thereon. The glass substrate 11 further has an anodized layer 113, a protective film 114 and a gate insulating film 115, which have been successively formed on the glass substrate 11 so as to cover the gate 12.
First, an amorphous silicon film (116) is deposited on the gate insulating film 115 by a chemical vapor deposition (hereinafter referred to as a "CVD") method.
Then, the amorphous silicon film (116) is crystallized by a laser crystallizing method using excimer laser light to form a poly-Si film 117.
Then, a silicon oxide film 118 is deposited on the poly-Si film 117 by a CVD method.
Thereafter, as shown in FIG. 6B, the silicon oxide film (118) is patterned by a lithography technique and etching process to form an etching stop pattern 119 of the silicon oxide film (118) on the poly-Si film 117 directly above the gate 112.
Then, as shown in FIG. 6C, an n-type impurity-containing silicon film 120 and a metal film 121 are successively deposited so as to cover the etching stop pattern 119 by a CVD method.
Thereafter, as shown in FIG. 6D, source-drain regions 122 and 123 are formed above the gate 112 and at both sides thereof from the metal film 121 and the silicon film 120 by a lithography technique and etching process.
Then, hydrogen (not shown) is introduced into the interface of the poly-Si film 117 in a region directly above the gate 112 through the etching stop pattern 119 by plasma hydrogenating treatment. Thus, dangling bonds are terminated by hydrogen and hence removed.
In this way, a TFT (Thin Film Transistor) 101 is formed.
The above-described TFT producing method and the TFT formed by the conventional method suffer, however, from the following problems:
Since hydrogen is supplied to the poly-Si film only through the etching stop pattern, it is difficult to supply hydrogen to the entire poly-Si film, and hence the resistance of the film becomes disadvantageously high.
Further, the activation annealing treatment for the source/drain regions must be carried out at high temperature.
In addition, since an electric field is likely to concentrate on a region in the vicinity of the drain region, the amount of leakage current is disadvantageously large.